UPDATE -
I got back on the phone with TT support today. The tech could not understand why the new map would not fit. She had me delete the current map and try to load the new map. There was still not enough room. Then she went to talk to a supervisor. She came back and she had me format the Tomtom. I did this reluctantly. She then had me copy the new map, the application, and the voice. She insisted that this would put the TT back to the exact state it would be in if it came from the factory. She said choose one voice file only. She was almost right. There was no start up screen, the doctor POIs were missing, and every time I started it, it came up that the voice that I selected was unavailable - and then had me select the computer voice again. I resolved that by loading the standard voices back. I had made a computer copy of the TT drive to my harddrive before I started any of this. I copied the startup screen file and the the doctor POI files directly to the TT. I also found that the sat images were not on the TT so I copied that file over also. At this point all seems to be working and all seems to fit with some room still to spare (but not much).
As to the SD card, I asked this tech the same question about the SD card. This time I was told, what I expected - and what everyone has said here - it is either or - if I use a card I must transfer the whole system over to that card. I asked what type of card and she said that it must be a standard SD card with a max of 2 GB.
Thanks to all.